Discharge lamp driving circuit

ABSTRACT

A discharge lamp driving circuit includes a chopper with a first switching circuit and an inverter with a second switching circuit. The chopper and inverter are connected to a dc voltage source and controlled to produce a composite lamp driving current composed of a high frequency alternating current interrupted by a dc current in order to keep the discharge lamp free from an acoustic resonance. The chopper and the inverter are arranged to have at least one common switching element in their first and second switching circuits.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a discharge lamp driving circuit,and more particularly to a circuit for operating a high-pressure gaseousdischarge lamp without causing a harmful acoustic resonance.

2. Description of the Prior Art

There has been a growing demand for a discharge lamp operating circuitwhich is operated at a higher frequency in order to reduce the weightand bulk of the ballasting inductor On the other hand, it is also knownthat discharge lamps, particularly high-pressure discharge lamps Such ismercury high pressure lamps and sodium vapor lamps suffer from unstabledischarge arcs due to "acoustic resonance" when operated at certain highfrequencies. U.S. Pat. No. 4,291,254 proposed to select a stablefrequency for avoiding such "acoustic resonance". However, such stablefrequency is seen only in a limited range and differs from differentkinds of lamps, thus reducing the flexibility of circuit design.Further, it is known that an extreme high frequency drive, for example,over 100 KHz may be effective for elimination of the "acousticresonance", but this eventually results in considerable switching lossesand noises which are not acceptable for the lamp operation. To this end,there has been proposed in Japanese Patent Publication (KOKAI) No.60-262392 to drive the lamp by a composite lamp driving Current having arepeating cycle of an alternating current interrupted by a dc current.This patent is based upon the findinq that the repetitive interruptionof the alternating current by the dc current can restrain the occurrenceof the "acoustic resonance", even the alternating current is in afrequency range which would otherwise cause the "acoustic resonance". Inthis sense, this patent is advantageous in selecting a lamp drivefrequency without having to consider the "acoustic resonance".Notwithstandinq this advantage, the patent has a certain drawback inthat two independent switching circuits, i.e., chopper and invertercircuits, are required for providing the dc current and the alternatingcurrent, respectively. This requires duplication of switching elementswith consequent complexity in incorporating the respective drivers incircuits for the duplicated switching elements, thus eventuallyresulting in increased cost and bulk of the physical circuit assembly.

SUMMARY OF THE INVENTION

The present invention eliminates the above problem by commonly utilizingat least one switching element for chopper and inverter switchingcircuits and provides an improved discharge lamp driving circuit withsimplified circuit arrangement.

It is therefore a primary object of the present invention to provide animproved discharge lamp driving circuit which is simple inconfiguration, yet preventing the acoustic resonance as well as assurinqto make compact the physical arrangement of the circuit.

An improved discharge lamp driving circuit in accordance with thepresent invention comprises a dc (direct current) voltage source,chopper means, and inverter means. The chopper means comprises a firstswitching circuit which is coupled to the dc voltage source to providetherefrom a periodically interrupted current and smooth the same forproducing a smoothed dc current. The inverter means comprises a secondswitching circuit which is also coupled to the dc voltage source forproducing therefrom a high frequency alternating current. Included inthe circuit is control means which is connected to the chopper means andthe inverter means in order to apply to the discharge lamp a repeatingcycle of a composite lamp driving current composed of the high frequencyalternating current supplied from the inverter means and is interruptedby the smoothed dc current supplied from the chopper means.

A characterizing feature of the present invention resides in that thefirst and second switching circuits of the chopper and inverter meansare arranged to have at least one common switching element whichoperates both in producing the dc current and the high frequencyalternating current. With this result, the circuit Configuration can beconsiderably simplified with consequent reduction in cost and bulk ofthe device.

In a preferred embodiment, the chopper means is configured in a bridgearrangement to apply to the discharge lamp the smoothed dc current whichis reversed in polarity from one cycle to the subsequent cycle of thecomposite lamp driving current. Thus, the deterioration of lampelectrodes can be reduced to a minimum, thereby giving rise to anenhanced life time of the discharge lamp.

The lamp driving circuit of the present invention can be broadlyclassified into two types with and without a transformer which operatesto apply the high frequency alternating current to the discharge lamp.In the former type, the first switching circuit of the chopper meanscomprises at least one switching element common to the second switchingcircuit of the inverter means. The common switching element is coupledin series circuit with an inductor and a parallel combination of thedischarge lamp and a bypass capacitor of which series circuit isconnected across the dc voltage source. During a first period of time,the common switching element is controlled to turn on and off at a firsthigh frequency to produce the interrupted current which is smoothed bythe first inductor and of which high frequency component is bypassedthrough the bypass capacitor for giving the smoothed dc current to thedischarge lamp within each cycle of the composite lamp driving current.The second switching circuit of the inverter means comprises a pair offirst and second switching elements at least one of which is common tothe first switching circuit of the chopper means. The first and secondswitching elements are connected in series across the dc voltage and isconnected in circuit with a dc blocking capacitor and the transformerwith a primary winding and a secondary winding which is inserted inseries relation with the discharge lamp and in parallel relation withthe bypass capacitor. The blocking capacitor is connected in series withthe primary winding of the transformer and in parallel with one of thefirst and second switching elements to form therewith a seriesoscillating circuit. During a second period of time alternating with theabove first period of time, the first and second switching elements arecontrolled to alternately turn on and off at a second frequency toprovide a high frequency alternating current in the series oscillatingcircuit as repeating to charge and discharge the dc blocking capacitor,whereby inducing the corresponding high frequency alternating current inthe circuit of the secondary winding and the discharge lamp to drive thedischarge lamp by such high frequency alternating current within eachcycle of the composite lamp driving current.

In the other type of the circuit without the transformer, the first andsecond switching circuits of the chopper and inverter means commonlyincludes a pair of first and second switching elements arranged in ahalf- or full-bridge configuration with a pair of capacitors or with apair of like switching elements. The bridge has its input ends connectedacross the dc voltage source and has its output ends connected across aseries circuit composed of an inductor and a parallel combination of thedischarge lamp and a bypass capacitor. During a first period of time,one of the first and second switching elements is controlled to turn onand off at a first frequency with the other switching element beinq keptturned off to provide a dc current which is smoothed by the inductor andis removed of its high frequency component by the bypass capacitor forfeeding the smoothed dc current to the discharge lamp within each cycleof the composite lamp driving current. During a second period of timealternating with the first period of time, the first and secondswitching elements are controlled to alternately turned on and off at asecond high frequency in such a manner as to provide to the dischargelamp said high frequency alternating current within each cycle of saidcomposite lamp driving current. The second high frequency is determinedto be lower than the first high frequency to such an extent that thesecond high frequency alternating current is supplied to the dischargelamp within each cycle of the composite lamp driving current whileallowing the second high frequency component to be substantially fed tothe discharge lamp without being bypassed through the bypass capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a discharge lamp driving circuitillustrating one basic version in accordance with the principle of thepresent invention;

FIG. 2 is a waveform chart illustrating a composite lamp driving currentin a conceptual form obtained in the present invention;

FIG. 3 is a circuit diagram of a discharge lamp driving circuit inaccordance with a first embodiment of the present invention;

FIG. 4 is a timing diagram of waveforms illustrating the operation ofthe two switching transistors shown in FIG. 3;

FIG. 5 is a waveform chart illustrating a composite lamp driving currentin a conceptual form obtained in the circuit of FIG. 3;

FIG. 6 is a circuit diagram of a modification of FIG. 3;

FIG. 7 is a circuit diagram of a second embodiment of the presentinvention;

FIG. 8 is a timing diagram of waveforms illustrating the operation ofthe four switching transistors shown in FIG. 7;

FIG. 9 is a timing diagram of waveforms illustrating another operationof the four switching transistors shown in FIG. 7 in accordance with amodification of the second embodiment;

FIG. 10 is a waveform chart illustrating a lamp driving current in aconceptual form obtained by the switching operation of FIG. 9;

FIG. 11 is a circuit diagram of a third embodiment of the presentinvention;

FIG. 12 is a timing diagram of waveforms illustrating the operation ofthe four switching transistors shown in FIG. 11;

FIG. 13 is a timing diagram of waveforms illustrating another operationof the four switching transistors shown in FIG. 11 in accordance with amodification of the third embodiment;

FIG. 14 is a circuit diagram of a fourth embodiment of the presentinvention;

FIG. 15 is a timing diagram of waveforms illustrating the operation ofthe two switching transistors shown in FIG. 14;

FIG. 16 is a waveform chart illustrating a composite lamp drivingcurrent in a conceptual form obtained by the switching operation of FIG.15;

FIG. 17 is a circuit diagram of a fifth embodiment of the presentinvention;

FIG. 18 is a timing diagram of waveforms illustrating the operation ofthe three switching transistors shown in FIG. 17;

FIG. 19 is a circuit diagram of a sixth embodiment of the presentinvention;

FIG. 20 is a timing diagram of waveforms illustrating the operation ofthe two switching transistors shown in FIG. 19;

FIG. 21 is a circuit diagram of a seventh embodiment of the presentinvention;

FIG. 22 is a circuit diagram of an eighth embodiment of the presentinvention;

FIG. 23 is a timing diagram of waveforms illustrating the operation ofthe two switching transistors shown in FIG. 22;

FIG. 24 is a circuit diagram of a ninth embodiment of the presentinvention.

FIG. 25 is a circuit diagram of a tenth embodiment of the presentinvention;

FIG. 26 is a timing diagram of waveforms illustrating the operation ofthe four switching transistors shown in FIG. 25; and

FIG. 27 is a timing diagram of waveforms illustrating another operationof the four switching transistors shown in FIG. 25 in accordance with amodification of the tenth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, one version of a discharge lamp driving circuitin accordance with the present invention is shown in a general form foreasy understanding of the principle of the present invention. Thecircuit comprises a switching section 1 which is connected across a dcvoltage source V₁ and includes at least two switching elements ortransistors collectively indicated in FIG. 1 as S. The switching section1 has a first pair of output ends 1a and 1b between which is connected aseries circuit of an inductor L₁ and a parallel combination of adischarge lamp 10 and a bypass capacitor C₁. Also provided at theswitching section 1 is a second pair of output ends 1c and 1d betweenwhich is connected a circuit of a dc blocking capacitor C₂ and atransformer T with a primary winding L₂₁, and a secondary windinq L₂₂.The secondary winding L₂₂ is coupled in series with the discharge lamp10 in parallel relation with the bypass capacitor C₁. The switchingsection 1 is controlled, during a first period of time T_(DC) (t₁ -t₂),to repetitively interrupt the dc voltage at a first high frequency forproviding a chopped voltage between the first pair of output ends 1a and1b. The chopped voltage is then smoothed by the inductor L₁ and has itshigh frequency component bypassed through the bypass capacitor C₁ tothereby feed a smoothed dc current I_(DC) to the discharge lamp 10during the first period T_(DC), as shown in FIG. 2 which shows acomposite lamp driving current Ia in a conceptual waveform. During asecond period of time T_(HF) (t₂ -T₃) alternating with the first periodof time T_(HF), the switching section 1 is controlled to repetitivelyinterrupt the dc voltage at a second high frequency in order to repeatproducing a voltage between output ends 1c and 1d and shorting the same.While the positive voltage is developed across the output ends 1c and1d, the voltage is applied through the dc blocking capacitor C₂ to theprimary winding L₂₁ of the transformer T so as to flow a current in onedirection in the primary winding L₂₁ as charging the blocking capacitorC₂. Upon subsequent shorting between the output ends 1c and 1d, theblocking capacitor C₂ discharges to feed an opposite current through theprimary winding L₂₁. consequently, the above repetition of developingthe voltage and shorting between the output ends 1c and 1d will inducean alternating current with the second high frequency at the secondarywinding L₂₂ of the transformer T such that the resulting high frequencyalternating current is caused to circulate through the closed loop ofthe lamp 10 and the bypass capacitor C₁ as it is blocked by the inductorL₁, whereby providing to the lamp the high frequency alternaing currentI_(AC) during the second period of time T_(HF), as shown in FIG. 2. Inthis manner, the switching section 1 provides a repeated cycle of acomposite lamp driving current composed of the high frequencyalternating current I_(AC) interrupted by the dc current I_(DC). Withthis result, the lamp 10 can be kept free from causing an acousticresonance or instable lamp operation even if the alternating current hasa high frequency which might cause such acoustic resonance when utilizedalone. The second frequency of the alternating lamp current may besuitably selected to be equal to or different from the first frequency awhich the switching section 1 is operated to chop the dc voltage. Withthe circuit arrangement of FIG. 1, it is possible to share at least oneswitching element in the switching section 1 which operates both in achopper mode for providing the dc current I_(DC) and in an inverter modeof providing the high frequency alternating current I_(AC). It is alsonoted in this connection that other components can be commonly andeffectively utilized in the above two different operation modes. Forexample, the bypass capacitor C₁, which acts to bypass the highfrequency component of the dc current in the chopper mode, serves tocomplete the closed loop with the secondary winding L₂₂ and thedischarge lamp 10 to effectively apply to the discharge lamp 10 thealternating voltage developed at the secondary winding L₂₂ in theinverter mode. Also the inductor L₁, which smooths the chopped voltagein the chopper mode, serves to block the high frequency voltage andprevent it from being applying to the circuit other than the closedloop, thereby applying the high frequency voltage effectively to thedischarge lamp 10 in the inverter mode. Further, the secondary windinqL₂₂ of the transformer T, which develops the high frequency voltage as apower source in the inverter mode, can serve to block the high frequencycomponent of the chopped voltage in the chopper mode, assisting tobypass the high frequency component through the bypass capacitor C₁. Thedischarge lamp 10 may be a high pressure gaseous discharge lamp such asmercury high pressure discharge lamp with metal halogen additives,sodium vapor lamp, and the like, or may be a low pressure lamp. Thepresent invention is now discussed in more detail with reference topreferred embodiments. Like numerals designate like parts throughout thefollowing embodiments for easy reference.

FIRST EMBODIMENT <FIGS. 3 to 5>

In this embodiment, the circuit comprises a pair of first and secondswitching transistors Q₁ and Q₂ which are cooperative with a pair ofcapacitors C₃ and C₄ to form a half-bridge having its input endsconnected across a dc voltage source V₁. Connected across the outputends of the of half-bridge is a series circuit composed of an inductorL₁ and the parallel combination of a discharge lamp 10 and a bypasscapacitor C₁. The first and second switching transistors Q₁ and Q₂ arecooperative with the series circuit to form a chopper which provides adc current to the discharge lamp 10. A transformer T is incorporated inthe circuit with its primary winding L₂₁ connected in series with a dcblocking capacitor C₂ across the second switching transistor Q₂ and withits secondary winding L₂₂ inserted in series with discharge lamp 10 andin parallel with the bypass capacitor C₁. The series connection of theprimary winding L₂₁ and the blocking capacitor C₂ is cooperative withthe first and second switching transistors Q₁ and Q₂ to form an inverteror series oscillating circuit which provides a high frequencyalternating current to the discharge lamp 10. As discussed in the below,the first and second switching transistors Q₁ and Q₂ are controlled toprovide repeating cycles of a composite lamp driving current composed ofthe dc current fed from the chopper and the high frequency alternatingcurrent from the inverter, as shown in FIG. 5. Typical values for theabove circuit are as follows. The voltage of the dc voltage source V₁ is280 V, the bypass capacitor C₁ has a capacitance of 0.22 μF, theinductor L₁ has an inductance of 0.2 mH, the primary winding L₂₁ has aninductance of 0.5 mH, and the blocking capacitor C₂ has a capacitance of0.1 pF. The first and second switching transistors Q₁ and Q₂ areoperated at 40 KHz both in the chopper and the inverter mode. Theoperation of the chopper is now explained with reference to FIG. 4.During each first period of time T_(DC) alternating with a second periodof time T_(HF), one of the first and second switching transistors Q₁ andQ₂ is controlled to turn on and off at a first high frequency while theother switching transistor is kept turned off. For example, during thefirst period of time T_(DC) (t₁ -t₂) of FIGS. 4 and 5, the firstswitching transistor Q₁ repeats turning on and off at a frequency of 40KHz while the second transistor Q₂ is kept turned off. When the firsttransistor Q₁ is on, the capacitor C₃ will discharge a current through aroute of the first switching transistor Q₁, secondary winding L₂₂,discharge lamp 10, and inductor L₁. When the first transistor Q₁ isturned off, the inductor L.sub. 1 acts to continuously flow the currentin the same direction. Thus, the dc current from the capacitor C₃ issmoothed by the inductor L₁ and has its high frequency componentbypassed through the bypass capacitor C₁ so as to feed the smoothed dccurrent to the discharge lamp 10. During the first period of time T_(DC)(t₃ -t₄), the second switching transistor Q₂ turns on and off at thesame frequency While the first switching transistor Q₁ is kept turnedoff, thereby producing the like dc current but in opposite polarity, asshown in FlG. 5. With this provision of reversing the polarity of the dccurrent from one cycle to the subsequent cycle of the composite lampdriving current, the discharge lamp 10 can have an elongated operationlife. The second period of time T_(HF) in which the inverter is activeto provide the high frequency alternating current is initiated bydriving to turn on and off the one of the switching transistor which iskept turned off in the previous first time of period while keeping theother switching transistor turning on and off. For example, in thesecond period of time T_(HF) (t₂ -t₃), the second switching transistorQ₂, which has been off in the previous first time of period T_(DC),begins to turn on and off while the first switching transistor Q₁continues to turn on and off. During this period, the first and secondswitching transistors Q₁ and Q₂ are alternately turned on and off with adead-time therebetween, in which both of the switching transistors aresimultaneously off, in order to provide the high frequency alternatingcurrent. The inverter operation in this period is explained in terms ofthe repeating sequence of the following four consecutive occurrences (1)to as indicated in FIG. 4.

At the first occurrence (1), the second switching transistor Q₂ isturned on while the first switching transistor Q₁ is off such that theblocking capacitor C₂ which has been charged due to the previous turningon of first switching transistor Q₁ will begin discharging to cause acurrent to flow through the primary winding L₂₁, second switchingtransistor Q₂, and back to the blocking capacitor C₂. At the secondoccurrence (2) in which both of the first and second switchingtransistors Q₁ and Q₂ are off, the primary winding L₂₁ in turn causesthe current to continuously flow through a first diode D₁, dc voltagesource V₁, blocking capacitor C₂ and back to the primary winding L₂₁. Atthe third occurrence (3), the first switching transistor Q₁ is on whilethe second switching transistor Q₂ is off so that the capacitor C₃discharges its energy, causing a current to flow in the oppositedirection through the first switching transistor Q₁, primary windingL₂₁, blocking capacitor C₂, capacitor C₄, and back to the capacitor C₃.At the fourth occurrence 4) where both of the first and second switchingtransistors Q₁ and Q₂ are off, the primary winding L₂₁ acts tocontinuously flow the current through a second diode D*, blockingcapacitor C₂, and back to the primary windinq L₂₁ in this sense, thefirst and second diodes D₁ and D₂, which are connected in antiparallelrelation respectively to the first and second switching transistors Q₁and Q₂, provide first and second bypass routes for continuously flowingthe instantaneous currents discharged from the primary windinq L₂₁ atthe second and fourth occurrences in which both of the switchingtransistors Q₁ and Q₂ are off. Likewise, in the next second period oftime T_(HF) (t₄ -t₅) which is initiated by turning on and off the firstswitching transistor Q₁ which has been on in the previous first time ofperiod T_(DC), the first and second switching transistors Q₁ and Q₂ arecontrolled to alternately turn on and off to produce the highalternating current through the primary winding L₂₁.

In this manner, during each second period of time T_(HF), the highfrequency alternating current continues to flow through the primarywinding L₂₁ to thereby induce at the secondary winding L₂₂ thecorresponding high frequency alternating current which circulatesthrough the closed loop of the secondary winding L₂₂, discharge lamp 10,and the bypass capacitor C₁ as the inductor L₁ acts to block such highfrequency alternating current, whereby driving the discharge lamp 10 bythus obtained high frequency alternating current, as shown in FIG. 5.

It is noted at this connection that the second frequency is determinedso that the inductor L₁ blocks such high frequency for circulating thehigh frequency alternating current through the closed loop of thesecondary winding L₂₂, discharge lamp 10, and bypass capacitor C₁. Dueto the above the circuit arrangement, the first and second switchingtransistors are commonly utilized both in the chopper mode of providingthe dc current and in the inverter mode of providing the high frequencyalternating current to the discharge lamp. In brief, the inverter modeof providing the high frequency alternating current is terminated bydeactivating one of the switching transistors, which in turn immediatelyinitiates the chopper mode of providing the dc current to the dischargelamp. Thus, by repeating the above operations, the high frequencyalternating current can be repetitively interrupted by the dc current,as shown in FIG. 5, to thereby inhibit the occurrence of the acousticresonance which would be otherwise develop due to the high frequencylamp driving.

The ratio of the period I_(DC) to the period of one complete cycle(T_(DC) +T_(HF)) of the composite lamp driving current may be Suitablyselected depending upon the kind of discharge lamp utilized forprevention of the acoustic resonance, as the suitable ratio will varywith different kinds of lamps.

Modification of the first embodiment <FIG. 6>

This modification shows a circuit arrangement which is identical to thatof FIG. 3 except that the dc blocking capacitor C₂ is inserted betweenthe primary winding L₂₁ and the connection of the capacitors C₃ and C₄.The circuit provides the like composite lamp driving current of FIG. 5by the like switching operations of FIG. 4, but in which each of thecapacitors Q₃ and Q₄ is additive to the blocking capacitor C₂ to act asa voltage source to provide the high frequency alternating currentduring the second period of Lime T_(HF). The other functions areidentical to those of the first embodiment. In other words, during thesecond period of time for providing the high alternating current, thecapacitors Q₃ and Q₄ can be made mainly responsible for producing thehigh frequency alternating current, while the blocking capacitor C₂ isresponsible for blocking the dc current. Accordingly, the constant ofthe blocking capacitor C₂ can be determined relatively freely withouttaking into account for utilizing it as the voltage source for thealternating current, thus giving rise to an improved design flexibilityof the circuit. Typical values for this circuit when operated on the dcvoltage V₁ of 280 V are as follows. C₁ =0.22 μF, L₁ =0.22 mH, L₂₁ =0.9mH, and C₂ =0.01 pF. The first and second switching transistors Q₁ andQ₂ are operated at 40 KHz and 60 KHz, respectively in the chopper modeand the inverter mode.

SECOND EMBODIMENT <FIGS. 7 and 8>

Referring to FIG. 7, a second embodiment of the present invention isshown to be identical to the first embodiment of FIG. 3 except forutilizing additional third and fourth switching transistors Q₃ and Q₄which are coupled with the first and second switching transistors Q₁ andQ₂ to form a chopper circuit of full-bridge configuration with third andfourth diodes D₁ and D₄ connected in antiparallel relation to the thirdand fourth switching transistors Q₃ and Q₄ respectively. Thus, as shownin FlG. 8, the one of the two diagonally disposed pairs of switchingtransistors Q₁, Q₄, and Q₂ Q₃ is rendered to be active while the otherpair is inactive during the first period of time T_(DC) providing the dccurrent to the discharge lamp 10. For example, during the first periodof time T_(DC) (t₁ -t₂), the first switching transistor Q₁ is controlledto turn on and off at a first high frequency with the fourth switchingtransistor Q₄ being kept turned on, while the second and third switchingtransistors Q₂ and Q₄ are kept turned off. In this manner the switchingtransistors are controlled to provide the dc lamp current which isopposite in polarity from the period t₁ -t₂ to the period t₃ -t₄. In theinverter mode of providing the high frequency alternating current, or inthe second period of time T_(HF) (t₂ -t₃ , t₄ -t₅), the first and secondswitching transistors Q₁ and Q₂ are controlled in the same manner as inthe first embodiment while the third and fourth switching transistors Q₃and Q₄ are kept turned off, during which the first and second switchingtransistors Q₁ and Q₂ are cooperative with the primary winding L₂₁ andthe blocking capacitor C₂ to oscillate an alternating current throughthe primary winding L₂₁ and the blocking capacitor C₂, as repeating thefour occurrences which are similar to those discussed with reference tothe first embodiment but differ in that the dc voltage V₁ will supply anenergy to the series oscillating circuit of the primary winding L₂₁ andthe blocking capacitor C₂ when the first switching transistor Q₁ is onand the second switching transistor Q₂ is off.

MODIFICATION OF THE SECOND EMBODIMENT <FIGS. 9 and 10>The modificationof the second embodiment utilizes the same circuit arrangement f FIG. 7,but operates the first and second switching transistors Q₁ and Q₂ asshown by the timing diagram of FIG. 9. As seen from FIG. 9, in thechopper mode of providing the dc current during the first period of timeI_(DC) (t₁ -t₂, t₃ -t₄), both of the first and second switchingtransistors Q₁ and Q₂ are operative to alternately turn on and off suchthat during this period T_(DC) the discharge lamp 10 receives, inaddition to the smoothed dc voltage from the dc voltage source V₁, theinduced voltage developed at the transformer T due to the oscillation inthe circuit of the primary winding L₂₁ and the blocking capacitor C₂.Thus, the resulting dc lamp current may take the form of FIG. 10 inwhich the high frequency component is superimposed on the smoothed dccurrent (t₁ -t₂, t₃, -t₄). THIRD EMBODIMENT <FIG. 11 and 12>

Referring to FIGS. 11 and 12, a third embodiment of the presentinvention is shown to comprise the same like circuit as in the secondembodiment except that the blocking capacitor C₂ has its one endconnected to the connection between the third and fourth switchingtransistors Q₃ and Q₄ The switching transistors Q₁ to Q₄ are controlledto turned on and off in accordance with a timing diagram of FIG. 12 toprovide the like composite lamp driving current as shown in FIG. 5 forthe first embodiment. In the chopper mode, the two diagonally disposedswitching transistors, for example, Q₁ and Q₄, are controlled to turn onand off in a synchronized manner while the other two switchinglransistors, for example, Q₂ and Q₃ are kept turned off during eachfirst period of time T_(DC) in order to provide the dc current to thedischarge lamp 10. Due to the synchronous operation of the diagonallydisposed switching transistors Q₁, Q₄ and Q₃, Q₄ in the chopper mode,the inductor L₁ acts upon the subsequent turning off of the allswitching transistors to cause the instantaneous dc current tocontinuously flow through a closed loop to the dc voltage supply V₁,thus feeding back the energy thereto. For example, when all of theswitching transistors are turned off immediately after the first andfourth switching transistors Q₁ and Q₄ being turned on, the inductor L₁cause the current to flow through the diode D₃, dc voltage source V₁,diode D₂, secondary winding L₂₂, discharge lamp 10 and back to theinductor L₂₁. On the other hand when all the switching transistors areturned off after the second and third switching transistors Q₂ and Q₃being simultaneously turned on, the instantaneous current from theinductor L₂₁ flows through the discharge lamp 10, the secondary windingL₂₂, diode D₁, dc voltage source, diode D₄, and back to the inductorL₂₁.

In the inverter mode of providing the high frequency alternating currentduring the second period of time T_(HF) (t₂ -t₃, t₄ -t₅), the twoswitching transistors, for example, Q₁ and Q₄ in one diagonally disposedpair are controlled to simultaneously turn on and off while theswitching transistors Q₂ and Q₁ in the other pair are controlled tosimultaneously turn on and off in an alternating manner therewith. Thus,the alternating current appears in the series oscillating circuitincluding the primary winding L₂₁ and the blocking capacitor C₂ with nosubstantial direct current being caused to flow through the dischargelamp 10 and the inductor L₁, whereby inducing at the secondary windingL₂₂ the high frequency alternating current which circulates the closedloop of the secondary winding L₂₂, discharge lamp 10, and bypasscapacitor C₁. Also in the inverter mode, when all the switchingtransistors are simultaneously turned off, the primary winding L₂₁ actsto fed back its accumulated energy to the dc voltage V₁ through thediodes D₃ and D₄ or the diodes D₁ and D₄.

MODIFICATION OF THE THIRD EMBODIMENT <FIG. 13>

A modification of the third embodiment utilizes the same circuit of FIG.11 but in which the switching transistors Q₁ to Q₄ are controlled in asomewhat different manner from the third embodiment. As shown in FIG.13, the difference is seen in that, during first period of time T_(DC)(chopper mode), the first switching transistor Q₁ is controlled to turnon and off while the fourth switching transistor Q₄ is kept turned on(t₁ -t₂), and the third switching transistor Q₃ is controlled to turn onand off while the second switching transistor Q₂ is on (t₃ -t₄). Thus,upon the turning off of the first switching transistor Q₁, the switchingtransistor Q₄ is cooperative to the diode D₂ to form a closed loopincluding the inductor L₁ for circulating therethrough the current whichis otherwise fed back to the dc voltage source V₁ through the diodes D₃and D₂ as seen in the third embodiment. Likewise, upon turning off ofthe third switching transistor Q₃, the second switching transistor Q₂ iscooperative with the diode D₄ to circulate the current which isotherwise fed to the dc voltage source V₁ through the diodes D₁ and D₄as seen in the third embodiment.

FOURTH EMBODIMENT <FIGS. 14 to 16>

A fourth embodiment of the present invention comprises first and secondswitching transistors Q₁ and Q₂ connected across the dc voltage sourceV₁ with first and second diodes D₁ and D₂ connected in antiparallelrespectively with the first and second switching transistors Q₁ and Q₂.The inductor L₁ is connected in series with a parallel combination ofthe discharge lamp 10 and bypass capacitor C₁, which series-parallelcombination is connected in parallel with the second switchingtransistor Q₂. Also included in the circuit is the transformer T ofwhich primary winding L₂₁ is connected in series with the dc blockingcapacitor C₂ across the second switching transistor Q₂. The secondarywinding L₂₂ is inserted in series with the discharge lamp 10 in parallelwith the bypass capacitor C₁. In this circuit, the chopper is defined bythe first switching transistor Q₁, and the series-parallel combinationof the inductor L₁, discharge lamp 10, bypass capacitor C₁, as enclosedin dotted lines CH in FIG. 14. Also, as enclosed in phantom lines IV inthe figure, the inverter is defined by, in additions to the abovecomponents common the to the chopper, the second switching transistor Q₂and the series circuit of the primary winding L₂₁ and the blockingcapacitor C₂, and the secondary winding L₂₂.

In operation, the first and second switching transistors Q₁ and Q₂ arecontrolled in a manner as shown in FlG. 15 to provide a composite lampdriving current of FIG. 16. That is, in the chopper mode during thefirst period of time T_(DC) (t₁ -t₂ and t₃ -t₄), the first switchingtransistor Q₁ is driven to turn on and off at a first high frequency toprovide to the chopped dc voltage which is smoothed by the inductor L₁and of which high frequency component is bypassed through the bypasscapacitor C₁ to flow the resulting smoothed dc current to the dischargelamp 10. In the inverter mode during the second period of time T_(HF)(t₂ -t₃), the first and second switching transistors Q₁ and Q₂ aredriven to alternately turn on and off with a dead-time therebetween at asecond high frequency, which may be equal to or different from the firsthigh frequency, to cause an oscillating current through the circuit ofthe primary winding L₂₁ and the blocking capacitor C₂ as repeating tocharge and discharge the blocking capacitor C₂ in the same manner aseffected in the previous embodiments. Thus, there is developed at thesecondary windinq L₂₂ the induced current which will circulate throughthe closed loop of the secondary winding L₂₂, discharge lamp 10, and thebypass capacitor C₁ as the inductor L₁ acts to block such high frequencyalternating current. In the inverter mode, the current flowing to theinductor L₁ is kept at a minimum by suitably selecting the values forthe inductor L₁, bypass capacitor C₁, the secondary winding L₂₂, and thedriving frequency or the second frequency at which the first and secondswitching transistors Q₁ and Q₂ are operated.

FIFTH EMBODIMENT <FIGS. 17 to 19>

The discharge lamp driving circuit comprises the first and secondswitching transistors Q₁ and Q₂ connected across the dc voltage sourceV₁ with first and second diodes D₁ and D₂ connected in antiparallelrelation respectively to the first and second switching transistors Q₁and Q₂. A third switching transistor Q₃ is connected in series with theinductor L₁ and the parallel combination of the discharge lamp 10 andthe bypass capacitor C₁ across the first switching transistor Q₁. Thetransformer T is connected in circuit with its primary winding L₂₁connected in series with the blocking capacitor C₂ across the secondswitching transistor Q₂ and with its secondary winding L₂₂ inserted inseries with the discharge lamp 10 and in parallel with the bypasscapacitor C₁. In this embodiment, the chopper is defined by the secondand third switching transistors Q₂ and Q₃, the series-parallel circuitof inductor L₁, discharge lamp 10 and bypass capacitor C₁, as enclosedin the dotted lines CH in FIG. 17, while the inverter is defined by, inaddition to the second switching transistor Q₂, the parallel combinationof discharge lamp 10 and bypass capacitor C₁ common to the chopper, thetransformer T and the blocking capacitor C₂, as enclosed in phantomlines IV in the figure. These switching transistors Q₁, Q₂, and Q₃ arecontrolled in accordance with a timing chart of FIG. 18 so as to providethe like composite lamp driving current as seen shown in FIG. 16 of thefourth embodiment. That is, in the chopper mode during the period T_(DC)(t₁ -t₂ and t₃ -t₄), the second switching transistor Q₂ is driven toturn on and off at a first high frequency while the first and thirdswitching transistors Q₁ and Q₃ are turned off and on, respectively,whereby providing the smoothed dc current to the discharge lamp 10 inthe same manner as in the fourth embodiment. In the inverter mode duringthe period T_(HF) (t₂ -t₃), the first and second switching transistorsQ₁ and Q₂ are driven to alternately turned on and off with a dead-timetherebetween at a second high frequency which may be equal to ordifferent from the first high frequency, in order to produce an highfrequency alternating current through the primary winding L₂₁ andconsequently circulate the resulting high frequency alternating currentthrough the closed loop of secondary winding L₂₂, discharge lamp 10, andbypass capacitor C₁, in the same manner as seen in the previous fourthembodiment. In the inverter mode, the third switching transistor Q₃ iskept turned off so that the discharge lamp 10 will not receive thecurrent directly from the blocking capacitor C₂ or from the dc voltagesource V₁.

SIXTH EMBODIMENT <FIGS. 19 and 20>

Referring to FIG. 19, a sixth embodiment of the present invention isshown to comprises the first and second switching transistors Q₁ and Q₂which are coupled with a pair of capacitors Q₃ and Q₄ in a half-bridgeconfiguration having its input ends connected across the dc voltagesource V₁. First and second diodes D_(1A) and D_(2A) are connected inantiparallel relation to the first and second switching transistors Q₁and Q₂, respectively. Included in the circuit is a diode networkcomposed of a series combination of diodes D₁₁ and D₁₂ and anotherseries combination of diodes D₂₁ and D₂₂. These series combinations areconnected in parallel with one another between the first and secondswitching transistors Q₁ and Q₂. Another first diode D_(1B) is connectedin series with diode D₂₁ in antiparallel relation to the first switchingtransistor Q₁. Likewise, another second diode D_(2B) is connected inseries with diode D₂₂ in antiparallel relation to the second switchingtransistor Q₂. A series circuit of a first inductor L₁ and the parallelcombination of discharge lamp 10 and bypass capacitor C₁ is connectedbetween the output ends of the half-bridge or between the connection ofdiodes D₁₁ and D₁₂ and the connection of capacitors C₃ and C₄. Alsoincluded in the circuit is the transformer T with its secondary windingL₂₂ connected in series with the discharge lamp 10 in parallel with thebypass capacitor C₁ and with its primary winding L₂₁ connected inparallel with a capacitor C₅. The parallel combination of the primarywinding L₂₁ and capacitor C₅ is connected in series with a secondinductor L₃ and a blocking capacitor C₂ between the connection ofcapacitors C₃ and C₄ and the connection of diodes D₂₁ and D₂₂. ln theabove circuit, the capacitors D₃ and C₄ are charged from the dc voltageV₁ and in return supplies to the discharge lamp 10 the like compositelamp driving current of FIG. 5 as the first and second switchingtransistors Q₁ and Q₂ are driven in accordance with a timing diagram ofFIG. 20.

In the chopper mode effected during the period T_(DC) (t₁ -t₂ and t₃ -t₄in FIG. 20), one of the first and second switching transistors Q₁ and Q₂is driven to turn on and off at a first high frequency while the otherswitching transistor is kept turned off, in order to provide across theoutput ends of the half-bridge the chopped voltage from one of thecorresponding capacitors C₃ and C₄. The chopped voltage is smoothed bythe first inductor L₁ and has its high frequency component routedthrough the bypass capacitor C₁, whereby applying the resulting smootheddc current to the discharge lamp 10. Such smoothed dc current has thepolarity which is reversed from the period of t₁ -t₂ to the period of t₃to t₄. The above chopper operation can be explained in terms of thefollowing repeated sequence. For example, in the period T_(DC) of t₁-t₂, when the first transistor Q₁ is on while the second switchingtransistor Q₂ is off, the capacitor C₃ supplies a current which flowsthrough first switching transistor Q₁, diode D₁₁, first inductor L₁,discharge lamp 10, secondary winding L₂₂, bypass capacitor C₁, and backto the capacitor C₃. Upon the subsequent turning off of the firstswitching transistor Q₁, the first inductor L₁ acts to continuously flowthe current through the discharge lamp 10, secondary winding L₂₂, bypasscapacitor C₁, capacitor C₄, diode D_(2A), and back to the first inductorL₁. During the period T_(DC) of t₃ -t₄, upon the turning on of thesecond switching transistor Q₂, the capacitor C₁, supplies a currentwhich flows through secondary winding L₂₂, discharge lamp 10, bypasscapacitor C₁, first inductor L₁, diode D₁₂, second switching transistorQ₂, and back to capacitor C₁. Upon the subsequent turning off of thesecond switching transistor Q₂, the first inductor L₁ causes the currentto continuously flow therefrom and through diode D_(1A), capacitorC.sub. 3, secondary winding L₂₂, discharge lamp 10, and back to thefirst inductor L₁. With provision of the half-bridge configuration, theload circuit including the discharge lamp 10 receive one-half of thevoltage of the dc voltage source.

For successfully bypassing the high frequency component of the choppedvoltage through the bypass capacitor Chd 1 without causing an unstablelamp operation and at the same time without requiring the capacitor C₁and the secondary winding L₂₂ to become bulky, the first high frequency,the inductor of the first inductor L₁, and the capacitance C₁ of thebypass capacitor C₁ may be selected such that the combined impedance ofthe lamp 10 and the secondary winding L₂₂ is to be 3 to 10 times that ofthe bypass capacitor C₁.

In the inverter mode during the period T_(HF) (t₂ -t₃ and t₄ -t₅), thefirst and second switching transistors Q₁ and Q₂ are controlled toalternately turn on and off with a dead-time therebetween at a secondhigh frequency to provide a high frequency alternating current to thedischarge lamp 10 as repeating the following four occurrences (1) to(4). At the first occurrence (1), for example, as indicated in theperiod of t₂ -t₃ in FIG. 20 in which the second switching transistor Q₂is turned on while the first switching transistor Q₁ is off, thecapacitor C₄ is cooperative with blocking capacitor C₂ to flow a currentmainly through primary winding L₂₁, capacitor C₅, second inductor L₃,blocking capacitor C₂, diode D₂₂, second switching transistor Q₂, andback to capacitor C₄. At the second occurrence (2) in which both of thefirst and second switching transistors Q₁ and Q₂, are simultaneouslyoff, the second inductor L₁ is cooperative with the primary winding L₂₁to release the accumulated energy to continuously flow the currentmainly through blocking capacitor C₂, diode D_(1B), capacitor C₃,primary winding L₂₁, capacitor C₅, and back to the second inductor L₃.At the third occurrence (3) in which the first switching transistor Q₁is on while the second switching transistor Q₂ is off, the capacitor C₃is cooperative with the blocking capacitor C₂ to flow a current in theopposite direction mainly through first switching transistor Q₁, diodeD₂₁, blocking capacitor C₂, second inductor L₃, primary winding L₂₁,capacitor C₅, and back to the capacitor C₃. At the fourth occurrence (4)in which both of the first and second switching transistors Q₁ and Q₂are off, the second inductor L₃ is cooperative with the primary windingL₂₁ to continuously flow the current mainly through primary winding L₂₁,capacitor C₅, capacitor C₄, diode D_(2B), blocking capacitor C₂, andback to the second inductor L₃.

In this connection, the diodes D_(1B) and D_(2B), which are connected inantiparallel relation respectively to the first and second switchingtransistors Q₁ and Q₂, provide first and second bypass routes forcontinuously flowing the instantaneous currents released from the secondinductor L₃ and the primary winding L₂₁ at the second and fourthoccurrences in which both of the switching transistors Q₁ and Q₂ areoff.

Likewise, in the net second period of time T_(HF) (t₄ -t₅) which isinitiated by turning on and off the first switching transistor Q₁ whichhas been active in the previous first time of period T_(DC), the firstand second switching transistors Q₁ and Q₂ are controlled to alternatelyturn on and off to produce the high alternating current through theprimary winding L₂₁.

In this manner, during each second period of time T_(HF), the highfrequency alternating current continues to flow the primary winding L₂₁to thereby induce at the secondary winding L₂₁ the corresponding highfrequency alternating current which circulates through the closed loopof the secondary winding L₂₂, discharge lamp 10, and bypass capacitor C₁as the inductor L₁ blocks such high frequency alternating current,whereby driving the discharge lamp 10 by thus obtained high frequencyalternating current.

It should be noted at this point that during the above inverter mode thefirst and second switching transistors Q₁ and Q₂ are driven at thesecond high frequency which is higher than the first high frequency atwhich they are driven in the above chopper mode, such that the firstinductor L₁ block the second high frequency to thereby allow only aminute current to divert into the circuit of first inductor L₁ andcapacitor C₁. It should be also noted that, during the chopper mode, theabove diode network of diodes D₁₁, D₁₂, D₂₁, and D₂₂ acts to prevent thecurrent from diverting into the inverter circuit of blocking capacitorC₂, second inductor L₃ , and primary winding L₂₁ since the diode networkacts to maintain the voltage of the blocking capacitor C₂ once it ischarged up to one-half of the voltage of the dc voltage source V₁.

To explain the diode network operation in detail with reference to theoccurrences during the chopper operation T_(DC), diodes D₁₂ and D₂₁ arecooperative to block the current from diverting into the invertercircuit when the first switching transistor Q₁ is on while the secondtransistor is off; diodes D₁₁, D₁₂, D₂₁, and D₂₂ are cooperative toblock the same when both of the first and second switching transistorsare off; and diodes D₁₂ and D₂₂ are cooperative to block the same whenthe second switching transistor Q₂ is on while the first switchingtransistor Q₁ is off.

With this arrangement of blocking the current from diverting into theinverter circuit during the chopper operation T_(DC), no substantialalternating current is induced at the secondary windinq L₂₂ to therebykeep the smoothed dc current free from any ripple which would otherwisebe superimposed thereupon in the absence of the diode network and wouldcertainly result in unstable lamp operation.

In this embodiment, the discharge lamp 10 may be ignited with theaddition of an L-C resonant starter circuit for inducing an increasedignition voltage across the secondary winding L₂₂ while providing thehigh frequency alternating current to the discharge lamp 10 by theoperation of the inverter. It is also effective to utilize apulse-width-modulation technique for controlling the output of thecircuit while monitoring the condition of the lamp 10 by means of thelamp current or the lamp voltage.

The period T_(HF) of the high frequency alternating current within onecycle (T_(HF) 30 T_(DC)) of the composite lamp driving current maydiffer from differing discharge lamps utilized, but is found, forexample, for a typical 80 W mercury-arc lamp having a rated lamp voltageof 115 V, to be preferably less than 20% of the on cycle in order toprevent the acoustic resonance and assure a stable lamp operation. Inthis instance, the one cycle (T_(HF) +T_(DC)) is preferably betweenseveral milliseconds and several tens of microseconds for the purpose ofrestraining flicker and noises.

The first and second switching frequencies can be suitably selected inrelation to the inductances and capacitances of the circuit. Forexample, when first inductor L₁, the bypass capacitor C₁, secondinductor L₃ blocking capacitor C₂, capacitor C₅ are selected to haverespective values that L₁ =0.2 mH, C₅ =0.1 μF, L₃ =0.2 mH, C₂ =0.15 μF,and C₅ =0.033 μF for driving the above mercury-arc lamp with the dcvoltage source V₁ of 280 V, the first and second switching transistorsQ₁ and Q₂ are operated respectively at 40 KHz during the period T_(DC)and respectively at 80 KHz during the period T_(HF) for providing thecomposite lamp driving current as indicated in FIG. 5.

SEVENTH EMBODIMENT <FIG. 21>

A seventh embodiment of the present invention has the similar circuitarrangement to that of the sixth embodiment except that the choppingoperation is effected only by the first transistor Q₁. That is, in thechopper mode, only the first switching transistor Q₁ is driven to turnon and off, while in the inverter mode, both of the first and secondswitching transistors Q₁ and Q₂ are driven to operate in the same manneras in the sixth embodiment, thus providing the composite lamp drivingcurrent as seen in FlG. 16 The like diode network composed of diodesD₁₁, D₁₂, D₂₁, and D₂₂ is also included to prevent the diversion of theundesired current between the series circuit of first inductor L₁,discharge lamp 10, bypass capacitor C₁, and secondary winding L₂₂ andthe inverter circuit of blocking capacitor C₂, second inductor L₃, andprimary winding L₂₁.

EIGHT EMBODIMENT <FIGS. 22 and 23>

Referring now to FIG. 22, an eighth embodiment of the present inventionis shown to comprise first and second switching transistors Q₁ and Q₂connected in series across a dc voltage source V₁ with first and seconddiodes D₁ and D₂ connected in antiparallel relation to the first andsecond switching transistors Q₁ and Q₂, respectively. The first andsecond switching transistors Q₁ and Q₂ are coupled with capacitors C₃and C₄ in a half-bridge configuration of which input ends are connectedto the dc voltage source V₁. Connected between the output ends of thehalf-bridge is a series circuit of an inductor L₁ and a parallelcombination of a discharge lamp 10 and a bypass capacitor C₁. The firstand second switching transistors Q₁ and Q₄ are controlled in accordancewith a timing chart of FIG. 23 to provide the like composite lampdriving current as shown in FIG. 5, as repeating the chopper andinverter functions.

In the chopper mode defined within the period T_(DC) (t₁ -t₂, t₃ -t₄) ofFIG. 23, one of the first and second switching transistors Q₁ and Q₂ isdriven to turn on and off at a first high frequency while the otherswitching transistor is kept turned off in order to provide a choppedvoltage supplied from the corresponding one of the capacitors C₃ and C₄.The chopped voltage is then smoothed by the inductor L₁ and of whichhigh frequency component is bypass through the bypass capacitor C₁, thusproviding the resulting smoothed dc current to the discharge lamp 10. Asapparent from FIG. 23, the first and second switching transistors Q₁ andQ₂ are alternately made active from one cycle to the subsequent cycle tothereby reverse the polarity of the dc current applied to the dischargelamp 10 in a repeated manner. Such polarity reversal is not essentialand therefore only one of the switching transistors Q₁ and Q₂ may bemade active in the chopper mode of providing the dc current.

In the inverter mode defined within the period T_(HF) (t₂ -t₃, t₄ -t₅),the first and second switching transistors Q₁ and Q₂ are driven toalternately turn on and off with a dead-time therebetween at a secondhigh frequency which is lower than the first high frequency, such thatthe discharge lamp 10 receives a resulting high frequency alternatingcurrent as the circuit repeats the following four occurrences (1) to(4), as indicated in FIG. 23. At the first occurrence (1) in which thefirst switching transistor Q₁ is off and the second switching transistorQ₂ is on, the capacitor C₄ discharges and causes a current to flowtherefrom mainly through the discharge lamp 10, inductor L₁, secondswitching transistor Q₂, and back to the capacitor Q₄. At the secondoccurrences (2) in which the first and second switching transistors Q₁and Q₂ are simultaneously off, the inductor L₁ acts to continuously flowthe current therefrom mainly through diode D₁, capacitor C₃, dischargelamp 10, and back to the inductor L₁. At the third occurrence (3) inwhich the first switching transistor Q₁ is on while second switchingtransistor Q₂ is off, the capacitor C₃ discharges and causes a currentto flow therefrom in the opposite direction through first switchingtransistor Q₁, inductor L₁, discharge lamp 10, and back to the capacitorC₃. At the fourth occurrence (4) when first and second switchingtransistors Q₁ and Q₂ are simultaneously off, the inductor L₁ act tocontinuously flow the current therefrom through discharge lamp 10,capacitor C₄, diode D₂, and back to the inductor L₁.

In the above circuit, the inductance of L₁ is selected to have such avalue that the second high frequency of the inverter operation will notbe lowered to audio frequency. In this connection, the first highfrequency of the chopper operation is selected to be higher than thesecond high frequency by such an extent as to increase the impedance ofthe inductor L₁ which limits the current flowing through the dischargelamp 10.

Typical values for the components of the above circuit are, forinstance, that C₁ =0.7 μF, L₁ =0.1 mH when V₁ =140 V and that Q₁ and Q₂are driven to operate at 100 KHz in the chopper mode and at 30 KHz inthe inverter mode.

NINTH EMBODIMENT <FIG. 24>

A ninth embodiment of the present invention utilizes a dc voltage sourcecomprising a diode bridge DB connected to an ac voltage source V_(AC)through a filtering circuit FT. The lamp driving circuit of the presentembodiment is similar to that of the eighth embodiment except that theoutput of the dc voltage source DB is connected across the firstswitching transistor Q₁ through a second inductor L₂. Thus, in thechopper mode, only the first switching transistor Q₁ is driven to turnon and off in order to provide the smoothed dc current to the dischargelamp 10 in the manner as described with reference to the eighthembodiment. The inverter operation of the circuit is identical to thatof the eighth embodiment. In this sense the first switching transistorQ₁ is commonly utilized both in the chopper and inverter modes. Thecharacterizing feature of the present embodiment resides in that, in thechopper mode, the first switching transistor Q₁ is cooperative with thesecond inductor L₂ and diode D₂ to act as a positive booster forincreasing the magnitude of the voltage acting to the series circuit ofcapacitors C₃ and C₄. That is, the energy accumulated in the secondinductor L₂ during the chopping operation is additive to the dc powersource DB to apply the resultant added voltage to the capacitors C₃ andC₄ for charging the same at the high frequency up to the voltage higherthan that of the ac voltage source V_(AC). Also with this result, theinput current I_(AC) can take the form of a sine wave, thereby making itpossible to have an improved power factor of more than 90%.

TENTH EMBODIMENT <FIGS. 25 AND 26<

A tenth embodiment of the present invention is similar to the eightembodiment except that another pair of switching transistors Q₃ and Q₄is incorporated as replacing the pair of capacitors C₃ and C₄ to form afull-bridge configuration with diodes D₃ and D₄ connected inantiparallel relation to the switching transistors Q₃ and Q₄. These fourswitching transistors Q₁ to Q₄ are driven in accordance with a timingchart of FIG. 26 to provide the like composite lamp driving current asshown in FIG. 5.

In the chopper mode defined within the period of T_(DC) (t₁ -t₂) duringwhich the second and third switching transistors Q₂ and Q₃ are off, thefirst switching transistor Q₁ is turned on and off at a first highfrequency while the fourth switching transistor Q₄ is kept turned on inorder to provide from the dc voltage V₁ a chopped voltage which issmoothed by the inductor L₁ and of which high frequency component isbypassed through the bypass capacitor C₁, thereby providing to thedischarge lamp 10 the smoothed dc voltage. Also in the chopper modedefined within the period of T_(DC) (t₃ -t₄) during which the first andfourth switching transistor Q₁ and Q₄ is off, the third switchingtransistor Q₃ is driven to turn on and off at the first frequency whilethe second switching transistor Q₂ is kept turned on, thereby providingthe smoothed dc voltage of the opposite polarity in the like manner asabove.

In the inverter mode defined within the period T_(HF) (t₂ -t₃), thefirst and fourth switching transistors Q₁ and Q₄ are driven to turn onand off at a second high frequency, which is lower than the first highfrequency, in synchronism with one another and in an alternate mannerwith the second and third switching transistors Q₂ and Q₃ turning on andoff in synchronism with one another, thus providing the resulting highfrequency alternating current to the discharge lamp 10 the highfrequency alternating current. In the inverter mode defined within theperiod T_(HF) (t₄ -t₅), these four transistors Q₁ to Q₄ are operated inthe opposite manner to provide the like high frequency alternatingcurrent to the discharge lamp 10. The second frequency, at which theswitching transistors Q₁ to Q₄ are operated to provide the highfrequency alternating current during the period T_(HF) (t₂ -t₃ and t₄-t₅), is selected such as to cause a resonance in the series circuit ofbypass capacitor C₁ and inductor L₁, enablinq to apply a high voltagesufficient to operate the discharge lamp stably. Typical values for thecomponents of the above circuit are, for instance, that C₁ =0.7 μF, L₁=0.1 mH when V₁ =140 V and that Q₁ and Q₂ are driven to operate at 100KHz in the chopper mode and at 30 KHz in the inverter mode.

MODIFICATION OF TENTH EMBODIMENT <FIG. 27>

The circuit of FIG. 25 of the tenth embodiment may be operated inaccordance with a timing chart of FIG. 27 for providing the likecomposite lamp driving current to the discharge lamp. In thismodification which is identical to the tenth embodiment except for theswitching operation in the chopper mode. That is, during the periodT_(DC) (t₁ -t₂), the first and fourth switching transistors Q₁ and Q₄are driven to simultaneously turn on and off, and during the periodT_(DC) (t₃ -t₄) the second and third switching transistors Q₂ and Q₃ aredriven to simultaneously turn on and off. With this result, upon thesimultaneous off of the paired transistors, the energy stored in theinductor L₁ in the previous turning on of the switching transistors isallowed to flow back to the dc voltage V₁ through the corresponding twoof the diodes D₁ to D₄. For example, when the switching transistors Q₁and Q₄ are simultaneously off after being turned on during the periodT_(DC) of t₁ -t₂, the inductor L₁ causes an instantaneous current toflow therefrom through diode D₃, dc voltage source V₁, diode D₂,discharge lamp 10, and back to the inductor L₁. And when the second andthird switching transistors Q₂ and Q₃ are off after being turned on, theinductor L₁ acts to continuously flow an instantaneous current inopposite direction therefrom through discharge lamp 10 and bypasscapacitor C₁, diode D₁, dc voltage source V₁, diode D₄ and back to theinductor L₁.

What is claimed is:
 1. A discharge lamp driving circuit comprising:a dc(direct current) voltage source; chopper means comprising a firstswitching circuit which is coupled to said dc voltage source to providetherefrom a periodically interrupted current and smooth the same forproducing a smoothed dc current; inverter means comprising a secondswitching circuit which is coupled to said dc voltage source forproducing therefrom a high frequency alternating current; control meansconnected to said chopper means and said inverter means to apply to saiddischarge lamp repeating cycle of a composite lamp driving currentcomposed of said high frequency alternating current interrupted by saidsmoothed dc current; said chopper means and said inverter means havingin their first and second switching circuits at least one commonswitching element.
 2. A discharge lamp driving circuit as set forth inclaim 1,wherein said chopper means is configured to apply said smootheddc current while reversing the polarity thereof from one cycle to thesubsequent cycle of said composite lamp driving current.
 3. A dischargelamp driving circuit as set forth in claim 1,wherein the first switchingcircuit of said chopper means comprises said at least one commonswitching element which is coupled in series circuit with an inductor,said voltage source, and a parallel combination of said discharge lampand a bypass capacitor, said switching element beinq driven to turn onand off at a first high frequency during a first period of time toproduce said interrupted current which is smoothed by said firstinductor and is removed of its high frequency component by said bypasscapacitor in order to apply said smoothed dc current to said dischargelamp within each cycle of said composite lamp driving current; thesecond switching circuit of said inverter means comprising a pair offirst and second switching elements at least one of which is common tosaid first switching circuit, said first and second switching elementsbeing connected in circuit with a dc blocking capacitor and atransformer with a primary winding and a secondary winding which isinserted in series relation with said discharge lamp and in parallelrelation with said bypass capacitor; said first and second switchingelements being connected in series circuit across said dc voltage sourcewith the series circuit of said dc blocking capacitor and the primarywinding being connected across one of said first and second switchingelements to form therewith a series oscillating circuit; said first andsecond switching elements being controlled to alternately turn on andoff at a second high frequency during a second period of timealternating with said first period of time in order to provide a highfrequency alternating current at said series oscillating circuit asrepeating to charge and discharge said dc blocking capacitor, wherebyinducing said high frequency alternating current in the circuit of saidsecondary winding and said discharge lamp to provide said high frequencyalternating current to said discharge lamp within each cycle of saidcomposite lamp driving current.
 4. A discharge lamp driving circuit asset forth in claim 1,wherein said first switching circuit of saidchopper means comprises a single pair of first and second switchingelements and a pair of first and second capacitors arranged in a halfbridge configuration having its input ends connected across said dcvoltage source and having its output ends connected across a seriescircuit of an inductor and a parallel combination of said discharge lampand a bypass capacitor; one of said first and second switching elementsbeing controlled to repetitively turn on and off at a first highfrequency while the other switching element is kept turned off during afirst period of time in order to provide to said discharge lamp withineach cycle of said composite lamp driving current the dc current whichis smoothed by said inductor and is removed of its high frequencycomponent by said bypass capacitor, said first and second switchingelements being controlled to alternately turn off at said first highfrequency with one of them being driven to repetitively turned on andoff so as to change the polarity of said smoothed dc current applied tosaid discharge lamp from one cycle to the subsequent cycle of saidcomposite lamp driving current; said second switching circuit of saidinverter means comprising a pair of first and second switching elementsat least one of which is common to said first switching circuit, saidfirst and second switching elements being connected in circuit with a dcblocking capacitor and a transformer with a primary winding and asecondary winding which is inserted in series relation with saiddischarge lamp and in parallel relation with said bypass capacitor; saidfirst and second switching elements being connected in series circuitacross said dc voltage source with the series circuit of said dcblocking capacitor and the primary winding being connected across one ofsaid first and second switching elements to form therewith a seriesoscillating circuit; said first and second switching elements beingcontrolled to alternately turn on and off at a second high frequencyduring a second period of time alternating with said first period oftime in order to provide a high frequency alternating current at saidseries oscillating circuit as repeating to charge and discharge said dcblocking capacitor, whereby inducing said high frequency alternatingcurrent in the circuit of said secondary winding and said discharge lampto provide said high frequency alternating current to said dischargelamp within each cycle of said composite lamp driving current.
 5. Adischarge lamp driving circuit as set forth in claim 4,wherein saidoscillating circuit includes first and second diodes; said first diodeconnected across said first switching element in antiparallel relationtherewith form a first bypass route for a first instantaneous currentwhich is discharged from said primary winding immediately after thesimultaneous turning off of said first and second switching elementsoccurring after the first and second switching elements beingsimultaneously turned off and on, respectively, whereby allowing saidfirst instantaneous current to continuously flow through saidoscillating circuit in one direction through said first bypass route;said second diode being connected across said second switching elementin antiparallel relation therewith to form a second bypass route for asecond instantaneous current which is discharged from said primarywinding immediately after the simultaneous turning off of said first andsecond switching elements occurring after the first and second switchingelements being simultaneously turned on and off respectively, wherebyallowing said second instantaneous current to continuously flow throughsaid oscillating circuit in the opposite direction through said secondbypass route.
 6. A discharge lamp driving circuit as set forth in claim5,further including a diode network connected in circuit between saidfirst switching circuit of said chopper means and said second switchingcircuit of said inverter means in such a manner as to prevent said dcblocking capacitor from repeating to be charged and discharged duringthe first period of time.
 7. A discharge lamp driving circuit as setforth in claim 1,wherein said first and second switching circuitscommonly includes a pair of first and second switching elements arrangedin a bridge configuration having its input ends connected across said dcvoltage source and having its output ends connected across a seriescircuit of an inductor and a parallel combination of said discharge lampand a bypass capacitor; one of said first and second switching elementsbeing controlled to turn on and off at first frequency with the otherswitching element being kept turned off during a first period of time toprovide to said series circuit a dc current which is smoothed by saidinductor and is removed of its high frequency component by said bypasscapacitor for feeding said smoothed dc current to said discharge lampwithin each cycle of said composite lamp driving current; said first andsecond switching elements being controlled to alternately turned on andoff during a second period of time alternating with said first period oftime in such a manner as to provide to said discharge lamp said highfrequency alternating current with a second high frequency within eachcycle of said composite lamp driving current.
 8. A discharge lampdriving circuit as set forth in claim 7,wherein said second highfrequency is lower than said first high frequency at which saidswitching elements are turned on and off for producing said smoothed dccurrent so that said high frequency alternating current at said secondfrequency is allowed to be substantially fed to said discharge lamp. 9.A discharge lamp driving circuit as set forth in claim 1,wherein saidfirst switching circuit of said chopper means comprises a pair of firstand second switching elements and a pair of first and second capacitorswhich are arranged in a half-bridge configuration having its input endsconnected across said dc voltage source and having its output endsconnected across a series circuit of an inductor and a parallelcombination of said discharge lamp and a bypass capacitor; each one ofsaid first and second switching elements being controlled to turn on andoff at a first high frequency with the other switching element beingkept turned off during first period of time to provide to said seriescircuit a dc current which is smoothed by said inductor and is removedof its high frequency component by said bypass capacitor in order toprovide said smoothed dc current to said discharge lamp while reversingthe polarity thereof from one cycle to the subsequent cycle of saidcomposite lamp driving current; said first and second switching elementsbeing controlled to turn on and off during a second period of timealternating with said first period of time to provide said highfrequency alternating current with a second high frequency to saidseries circuit of said inductor and the parallel combination of saiddischarge lamp and said bypass capacitor; said second high frequencybeing lower than said first high frequency to such an extent that saidsecond high frequency alternating current is supplied to said dischargelamp within each cycle of said composite lamp driving current whileallowing said second high frequency component to be fed substantially tosaid discharge lamp without being substantially bypassed through saidbypass capacitor.